The present invention relates to sensors for detecting changes in a variety of conditions, including temperature, chemical conditions including pH changes and levels of specific ions, levels of biological antigens, radiation levels, and electrical field strength. Sensors for detecting these parameters are based on the principle wherein the integrated intensity of light (or other wave energy) in the vicinity of a light source, diffused and scattered within an optical scattering medium, will increase as the effective concentration of the scattering centers within the medium increases, i.e., as the average distance between scattering centers decreases or as the scattering centers change their light-scattering properties. The sensors and detectors of the present invention may be used in a wide variety of applications, including laboratory and clinical instrumentation as well as industrial uses in a variety of applications wherein detection of these parameters is required.
The present invention relies on the principle whereby the intensity of light or other wave energy from a source, which is diffused and scattered within a scattering medium such as translucent foam, is increased in the vicinity of the light source as the effective concentration of scattering centers within the medium increases, i.e., the average distance between scattering centers decreases or their light-scattering properties are enhanced.
The region within the medium which contains scattered light from the source is known as a xe2x80x9cvirtual optical cavityxe2x80x9d, in that the properties of an optical cavity are emulated. For convenience, the term xe2x80x9coptical cavityxe2x80x9d will refer to a virtual optical cavity. The intensity of scattered light at any particular position within an optical cavity is referred to as the xe2x80x9cintegrated intensityxe2x80x9d of the light at that position. Thus, as the medium is compressed by, for example, the application of pressure, the integrated intensity of the light within the region immediately surrounding the light source increases in intensity. The increase is proportional to the increase in concentration of scattering centers. This in turn may be related increases in pressure applied to the medium. A consequent decrease in light intensity occurs within a more distant region within the medium. For example, U.S. patent application Ser. No. 08/895,268 (Reimer et al.) describes a pressure sensor based on this principle, in which the scattering medium may comprise either a material having scattering centers dispersed generally evenly therein, or a hollow deformable container, the inner surface of which diffuses light or other wavelike energy directed into the medium. The light source forms an integrated cavity within the medium, defined by a region containing fully scattered light from the source. When pressure is applied to the medium, the medium compresses and increases the concentration of scattering centers in the region surrounding the light source. The resulting increase in light intensity is detected by a receptor and communicated to an information processor. In one version, a multiplicity of light sources and receivers permits the general location of the pressure to be resolved. Within an apparatus of this type, one or more light sources and detectors are provided, with each source and its corresponding detector being generally adjacent to each other or close together. Most conveniently, the scattering medium comprises a compressible, translucent material such as plastic foam. An array of source/detectors pairs may be provided to provide localized pressure detection means. The detector or detectors are associated with a signal processing unit, which receives information from the detectors corresponding to the detected integrated light intensity levels, and resolves this information into a corresponding pressure level experienced by the scattering medium.
It has not been previously proposed to provide an apparatus which makes use of an integrating optical cavity of the above type for the detection of parameters other than pressure. Thus, any phenomenon, condition or parameter which increases the concentration of scattering centers within a scattering medium, may be detected by means of detecting an increase or decrease in integrated light intensity within the scattering medium, wherein the light is provided by a source of known intensity.
It has been observed that polymer gels, which are generically referred to as hydrogels, can be adapted to serve as a light-scattering medium. An example of such a gel is polyacrylamide. These gels can be engineered to swell or shrink in response to specified chemical or physical changes in their environment. For example, a hydrogel graft copolymer of PMMA (polymethylmethacrylate) and PEG (polyethylene glycol) is pH sensitive. Carboxylic acid groups of the PMMA tend to be protonated at low pH. Hydrogen bonds form between carboxylic acid groups and the ether oxygens on the PEG chain. This reversible ionic polymerization increases the hydrophobicity of the polymer matrix, and water is expelled from the gel and the matrix tends to shrink or collapse. The total resulting volume change can be large, as much as a factor of 1000. If the collapsed gel is exposed to high pH values, the carboxylic acid group become ionized, and the gel becomes more hydrophillic; it then absorbs water and expands. Examples of such a gels have been described by Tanaka in U.S. Pat. Nos. 6,030,604; 5,801,221; 5,242,491; 4,732,930. It is also feasible to provide gels within which functional groups are contained.
It will be understood that within this specification, references to the term xe2x80x9clightxe2x80x9d apply to other wave energy sources, including sound and non-visible electromagnetic radiation, with suitable and obvious modifications to the apparatus and method embodiments described herein.
An object of the present invention is to provide an improved sensor which may be adapted to detect levels of various physical phenomena, including pH, temperature, ion level, radiation, electrical field strength, and biologically active antigens.
In one aspect, the invention comprises a sensor for detecting various physical or chemical changes within the environment surrounding the detector. In this aspect, the invention comprises a light source, translucent medium having light scattering centers dispersed therein to fully scatter light from the light source, a detector positioned in the vicinity of the source responsive to the integrated intensity of light from the light source, and an information processing means in communication with the detector for translating the detected light intensity into an electronic signal responsive to the level of a selected physical or chemical parameter. The translucent medium and/or the scattering centers are selected to respond to changes in various physical or chemical parameters by altering either the concentration or reflective properties of the scattering centers in the region surrounding the light source. This region is referred to herein as an xe2x80x9coptical cavityxe2x80x9d, and it is understood that this is defined by a region within the translucent material in the vicinity of the light source, within which light from the light source is fully scattered and an increase in the concentration of scattering centers produces a measurable increase in the integrated intensity of light. Outside the cavity, the intensity of scattered wave energy decreases as the dimension of the cavity decreases. The boundary of the cavity is related to the characteristic scattering length of the medium. Typically the interior of the cavity will be less then one characteristic scattering length removed from the energy source whereas the exterior of the cavity will be more then one characteristic scattering length removed from the energy source.
In one version, the invention comprises a temperature sensor having an electronic output responsive to the temperature of a selected medium. In this version, the scattering medium comprises a solid material such as opal glass, polyethylene or a transparent polymer such as PMMA (polymethyl methacrylate) or a hydrated gel polymer such as polyacrylamide, with an embedded scattering material such as particles of titanium dioxide evenly disbursed within the material. Fluctuations in temperature result in corresponding expansion or contraction of the solid material resulting in a corresponding change in the concentration of the scattering agent within the material. This causes a corresponding increase or decrease in the integrated intensity of light within the integrating cavity. As the coefficients of expansion are relatively small within material of this type, this type of sensor is advantageous for wide temperature ranges. The integrated light intensity is then detected by detection means, communicated to a signal processing means, and converted into an electronic signal.
In a further aspect, a temperature sensor may be based on a temperature-mediated change in the light-scattering properties of a scattering medium. In this version, a light emitter/detector pair is embedded in a translucent material, characterized by a polycrystalline phase transition in the temperature range of interest. Liquid crystals (nematic) are a common example of this type of material. This medium is characterized by reversible temperaturexe2x80x94dependent changes in the concentration of crystalline structures within the medium. Crystallization within the medium increases the light scattering in the material. An increase in the temperature of the medium causes a corresponding decrease in concentration of scattering crystal. This in turn results in a corresponding decrease in integrated light intensity within the cavity. The same effects can be achieved using a hydrated gel that passes through a phase transition within a specified temperature range. A sensor of this type is characterized by high sensitivity, within a relatively narrow temperature range.
In a further aspect, a sensor for detection of pH levels comprises a volume of a scattering medium. The scattering medium is comprised of a hydrated polymer gel matrix, within which light scattering particulates such as titanium dioxide particles are evenly dispersed and trapped in the gel as stable light scattering centers. Functional groups on the gel are treated to react over a pH range of interest. It is established in the scientific literature that polymer gels such as a hydrogel graft copolymer of PMMA and PEG may be engineered to swell or shrink in response to specified chemical or physical changes (e.g. see Tanaka, Scientific American, January 1981). As the gel deforms in response to changing pH or other physical or chemical influences, the scattering center concentration undergoes a consequent change, thereby causing a consequential increase or decrease in the integrated light intensity within the optical cavity. A light emitter/detector pair is embedded within the matrix forms an optical cavity within the medium. The integrated intensity of light within the cavity is detected by detector means. Signals from the detector are processed as described above, to generate an electronic signal corresponding to the pH of the medium. In a similar fashion, the sensor may be adapted to respond to other chemical conditions. In particular, the polymer gel may be engineered to respond by expansion or contraction to other chemical conditions such as levels of specific ions.
In a further aspect, a sensor detects levels of specific biologically-active antigens, taking advantage of the molecular biological phenomenon of antibody/antigen reactivity. In this version, an emitter/detector pair is embedded within a scattering medium comprising a hydrated gel matrix, within which functional groups are affixed or embedded which include an affixed immune reagent (or enzyme) with specificity for a designated bio-organic molecule. When the immune reagent binds to the reactant in an antigen/antibody specific reaction, the scattering co-efficient of the immune reagent increases, thereby causing a change in the light intensity within the optical cavity within the gel matrix. The change in intensity is detected by a photoreceptor, which in turn communicates the information to the central processing unit.
In a further aspect, a sensor detects electromagnetic radiation levels. In this version, a radiation sensitive reactant such as bis(4-dimethylaminophenyl)4-vinylphenylmethane-leucocyanide (see for example Tanaka, U.S. Pat. No. 5,242,491) is dispersed throughout a scattering medium. This comprises a photosensitive gel which expands or contracts in response to light exposure. A light emitter and detector is implanted within or in operative association with the medium. The reactant may be adapted to detect visible or near-visible light, ionizing radiation or other forms of electromagnetic radiation. In the case of light, a photo reaction within the reactant in response to light exposure creates scattering centers within the medium. This in turn scatters light emanating from the light emitter, forming an optical cavity within the medium. The detector detects the light intensity within the cavity, which increases as the ambient light level increases. For detection of ionizing radiation, a scattering medium such as glass or PMMA may be provided which is devoid of scattering centers. Exposure to ionizing radiation results in damage to the integrity of the material, with the resulting dislocations and defects acting as scattering centers, which in turn increase the integrated intensity of light within the optical cavity.
In a further aspect, a sensor detects the presence or absence of an electrical field. In this version, a hydrated polymer gel matrix having scattering particulates dispersed therein comprises the scattering medium. The gel incorporates functional groups sensitive to the presence of an electric field. Field sensitivity causes the gel to shrink or swell, thereby changing the effective density of the scattering particulates, thereby changing the integrated light intensity within the optical cavity.
In a further aspect, sensors combine various of the detection means described above. For example, a combined pressure and temperature sensor may be made by providing a scattering medium, within which an emitter/detector pair is positioned, along with a second detector outside the optical cavity. The scattering particulates are coated with a thermochromic substance such as 4,4xe2x80x2-bis(4xe2x80x3,4xe2x80x3-(N,N-diethylamino)styryl)-2,2xe2x80x2-bipyridine, which is one example of a large class of commercially available thermochromic paints or inks available from Chromophore, Hallcrest and other suppliers, which changes its optical absorption characteristics in response to temperature changes. In one version, the medium may be an open cell polyurethane foam, 50% compressible in the pressure range of 100 Pa to 10,000 Pa, and coated with a thermochromic paint sensitive in the temperature range from 35xc2x0 C. to 40xc2x0 C. This embodiment would have a pressure/thermal sensitivity quite similar to human skin. In this version, the second photodetector enables separate discrimination of absorption effects and cavity deformation effects, thereby permitting the processing unit to discriminate between light intensity changes within the optical cavity caused by temperature changes and those caused by the application of pressure to the medium.
Having thus generally characterized the invention, a detailed description of preferred embodiments of the invention will follow, by way of reference to the attached drawings wherein: